PRAMANA c© Indian Academy of Sciences Vol. xx, No. x— journal of Month 2014

physics pp. 1–4

Observation of two-photon absorption at UV radiationin ZnS quantum dots

MANAJIT CHATTOPADHYAY1, PATHIK KUMBHAKAR1,∗ andUDIT CHATTERJEE2

1Nanoscience Laboratory, Department of Physics, National Institute of Technology,Durgapur 713 209, India2Laser Laboratory, Department of Physics, Burdwan University, Burdwan 713 104, India∗Corresponding author. E-mail: kumbhakarp@yahoo.com; pathik.kumbhakar@phy.nitdgp.ac.in

DOI: 10.1007/s12043-014-0685-x

Abstract. Research studies on quantum dots (QDs) of semiconductor materials are of potentialinterest in present days having promising applications in different optoelectronic devices. Amongother materials, ZnS is a direct bandgap semiconductor material having a wide bandgap of 3.6 eVfor its cubic phase at room temperature and it shows excellent optical properties. However, here thenonlinear optical (NLO) properties of chemically synthesized ZnS QDs of average size of ∼1.5 nmhave been reported which are measured by using an indigenously developed Z-scan technique. Thepump radiation is 355 nm which is the third harmonic of the Q-switched Nd:YAG laser radiationhaving pulsed duration of 10 ns with the repetition rate of 10 Hz. The measured experimental datahave been analysed by using analytical models and two-photon absorption coefficients of the ZnSQDs at 355 nm have been extracted.

Keywords. Nonlinear optical properties; quantum dots; Z-scan.

PACS Nos 42.65.–k; 78.67.Bf

Research interest has witnessed an exponential growth in recent times to find highlypotential multiphoton active materials [1–10]. Quantum dots (QDs) of zinc sulphide(ZnS) and Mn2+-doped ZnS (ZnS:Mn) have been found to be potential materials due totheir visible photoluminescence and enhanced nonlinear optical (NLO) properties [3–6].There are some previous reports on the multiphoton absorption properties of ZnS andZnS:Mn QDs, viz. two-photon absorption (2PA) has been reported by Zheng et al [5] at532 nm radiation with picosecond laser radiation in 0.5% Mn2+-doped ZnS QDs. Also,some enhancement in 2PA have been reported in ZnSe/ZnS core/shell and ZnS quantumstructures at 806 nm and 532 nm laser pulses, respectively [8]. NLO properties of ZnS inthe ultraviolet (UV) wavelength region have not been investigated extensively. However,in this work we have presented our observation of 2PA in undoped ZnS QDs at 355 nm

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Manajit Chattopadhyay, Pathik Kumbhakar and Udit Chatterjee

Figure 1. Normalized transmittance curves for OA Z-scan at 355 nm incident laserradiations for ZnS QDs sample at 27.4 GW/cm2 peak intensity. Symbols are exper-imental points. The solid curve is the theoretically fitted one with 2PA processes inthick media. Left inset shows UV–visible absorption characteristics of ZnS sampledispersed in methanol [6]. Right inset shows the wavelength dispersion of the normal-ized 2PA coefficients for bulk semiconductor considering 2PA scaling rule as reportedby Sheik-Bahae et al [10]. Red, blue, and magenta curves correspond to the normal-ized 2PA coefficients calculated by taking the band-gap values as 4, 5.2, and 6 eV,respectively [10]. It has been found that the normalized 2PA shows a peak at 355 nmfor Eg = 5.2 eV (this is shown by a vertical line).

UV radiation. The sample used in this work is freshly prepared by following the chemi-cal precipitation method as elaborated in [6]. UV–visible absorption characteristic of thesample, measured by a spectrophotometer (Hitachi U-3010) are shown as the left inset offigure 1 [6]. The average particle size (radius) as obtained from X-ray diffraction (XRD)measurements is ∼1.5 nm [6] and a standard Z-scan experimental set-up is used [6,10].However, in the present experiment, the third harmonic radiation generated at 355 nm (10ns, 10 Hz rep. rate) has been used as the excitation source. This third harmonic UV radi-ation from a Q-switched Nd:YAG laser having 10 ns pulse duration and 10 Hz repetitionrate is allowed to pass through a quartz lens of 7.5 cm focal length and then focussed onthe sample (ZnS nanopowder dispersed in methanol and kept in a quartz cuvette of 2 mmpath length (L)). This satisfies the thick sample condition (L > n0z0) [9], where n0 andz0 are the refractive index of the sample and the confocal parameter of the used Z-scansystem respectively [9].

In the present experiment, for thick sample condition, the on-axis normalized openaperture (OA) transmittance (TOA) can be written as [9],

TOA = [1 + 0.5Q0[tan−1(ZN + l)− tan−1(ZN)

]−1, (1)

where ZN = z/n0z0, l = L/n0z0,Q0 = q0[1 + tanh(l/2) × {(3ϕ0/10) + (ϕ20/8)}],

q0 = β2PAI0n0z0, and ϕ0 = 2πγn0z0/λ. Here, I0 is the on-axis peak irradiance at focus

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Two-photon absorption at UV radiation

in W/cm2, and β2PA and γ are the 2PA coefficient and nonlinear refraction coefficient ofthe sample, respectively [9]. Here λ is the wavelength of the used radiation. Figure 1shows our experimentally and theoretically fitted OA Z-scan transmission traces for ZnSQDs sample measured with the maximum intensity of 27.4 GW/cm2. Here, symbols areexperimental points and the solid curve is the theoretically fitted one using eq. (1). Fromfigure 1 it is obvious that the theoretical curve obtained by considering 2PA showsexcellent fittings to the experimental data.

The observed NLO properties are endorsed only to the semiconductor QDs sampleand the quartz cuvette filled with methanol did not show any NLO properties at thiswavelength. The value of intrinsic 2PA coefficient (β Int

2PA) of the used sample is calcu-

lated using the relation β Int2PA = [

β2PA(nsoln0 )2

]/[(n

QDs0 )2f (£) 4

], the details of which have

already been given in [3]. The obtained value of 2PA coefficient (β Int2PA) is given against

values from other literature in table 1. The value of incident photon energy (Ep) is ∼3.5eV and the band-gap energy (Eg) of the ZnS QDs sample is ∼5.16 eV which satisfies thecondition 2Ep > Eg. It clearly shows the dominance of 2PA in the present experiment[4,5]. In another calculation, using the dispersion scaling rule given by Sheik-Bahae et al[10], it has been found from inset (right) of figure 1 that at the used wavelength of 355 nm,the normalized 2PA is maximum (peak) when the band gap of the material and the wave-length of the photon energies are taken as 5.2 eV and ∼3.5 eV, respectively. In figure 1,red, blue, and magenta curves showing the variation of the normalized 2PA coefficientscalculated by taking the different values of bandgaps as 4, 5.2 and 6 eV, respectively.At Eg = 5.2 eV, the blue curve shows the maximum (peak) normalized 2PA at 355 nm,whereas the other curves (i.e. for different band gaps) show the deviation of peak maximafrom 355 nm, indicating the higher probability of occurrence of 2PA at 355 nm at theband-gap energy (Eg) of 5.2 eV. So a suitable choice of the band gap of semiconductoras well as the corresponding wavelength is the best combination for obtaining large 2PA.The average particle size of 1.5 nm of the used QDs being less than the exciton Bohrradius of 2.2 nm of bulk ZnS, the huge enhancement in 2PA coefficient is attributed to thequantum confinement effects as well as to the large surface-to-volume ratio of the QDs.

In summary, we have presented here, 105 times enhanced 2PA coefficient in ZnS QDsover that of bulk ZnS at 355 nm by Z-scan technique [7]. This huge enhancement in2PA in the studied sample is attributed to the quantum confinement effects. As the parti-cle size is reduced below the exciton Bohr radius of the material, there is concentration

Table 1. 2PA coefficient measured in ZnS QDs at 355 nm wave-length at 27.4 GW/cm2 intensity.

Sample βInt (cm/GW)

This work Literature data

ZnS QDs 3.8 × 105 1a, 1.18b, 1 ± 0.3c

aBulk ZnS at 610 nm [7].bZnS QDs at 532 nm [5].cZnS QDs at 795 nm [8].

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Manajit Chattopadhyay, Pathik Kumbhakar and Udit Chatterjee

of oscillator strength into just a few transitions and this leads to the enhancement in thenonlinearity of the material. Moreover, the productive combination of the experimentalparameters, such as the incident photon energy and the band gap of the used samples, pro-vided the highest probability for achieving 2PA in the investigated samples. The presentobservation of enhanced nonlinear absorption of the undoped ZnS sample at 355 nm,combined with our previous observations at 532 and 1064 nm [3,6] showed that this QDis excellent to be used in optoelectronic and biophotonic devices [2–6].

Acknowledgements

Authors are grateful to DST (Grant No. SR/FTP/PS-67/2008), Govt. of India, for thepartial financial support.

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